Lithographic or planographic printing employs an offset printing plate to produce printed images. Such a plate may be a sheet of paper, plastic base or other suitable substrate which has a hydrophobic image region and a hydrophilic nonimage or background region. The image area is wetted during the printing process by a lithographic printing ink, while the hydrophilic background (nonimage) areas repel and are not wetted by the ink. Typically, the offset printing plate is formed from an electrophotographic master which has a light-sensitive layer comprising fine particles of a photoconductive material (e.g., zinc oxide) dispersed in a resin binder. The electrophotographic master is subjected to a conventional process to form a hydrophobic image on the light-sensitive layer.
After the completion of the electrophotographic operation, the master is transformed into a crude offset printing plate, or master copy sheet, comprising a somewhat water wettable nonimage area (hydrophilic area) and a poorly water wettable image area (hydrophobic area). However, since the entire surface of the electrophotographic master is originally a hydrophobic photoconductive layer, even after completion of the electrophotographic operation, the background (non-image) areas on the plate initially remain partially hydrophobic in nature. Thus, if the plate were used immediately, the printing ink would also adhere to the nonimage, background areas, resulting in unsatisfactory quality of the printed products. It is therefore necessary to render the nonimage areas of the master hydrophilic (and to remove any hydrophobic properties) by "converting" or "desensitizing" it before printing.
Desensitizing or conversion solutions (also called "etching solutions") that have been proposed to date may be classified into two types, one type being a cyanide compound containing solution, usually comprising a ferrocyanate (as described in U.S. Pat. No. 3,001,872) or ferricyanate, and the other type being a cyanide-free solution, usually comprising an amine cobalt complex (as described in U.S. Pat. No. 4,208,212), pnytic acid (as described in U.S. Pat. No. 3,592,640 and Japanese Patent Publication No. 2839/83) or its derivative, or a guanidine derivative.
However, none of the aforementioned processing solutions have been found to be completely satisfactory. Although the first type of solution containing a ferricyanate or ferrocyanate has the advantages of high desensitizing power, ability to form a strong hydrophilic film, and fast film formation, such a cyanide-containing solution suffers from several significant problems. Because the ferrocyanide or ferricyanide ion is somewhat photosensitive and heat sensitive, when it is exposed to light, it undergoes either a color forming reaction or a precipitation reaction that weakens the desensitizing power of the solution. More significantly, however, the cyanide ions present in such formulations lead to the formation of detectable free cyanide in the waste effluent from the conversion process which, if not properly treated, may pollute water-courses and cause other environmental concerns.
Two examples of this type of cyanide-containing solutions are provided by U.S. Pat. Nos. 3,764,353 and 3,211,686. The '353 patent discloses a desensitizing, or conversion, solution that contains a ferrocyanide or ferricyanide salt, a polybasic organic or inorganic acid such as succinic acid, a polyvalent metallic salt such as salts containing phosphate, and a chelating agent. The chelating agent assists in retaining metallic ions, such as inorganic phosphates, in solution. The agent accomplishes the solubilization by chelating with the metallic ions, and thus preventing precipitation of compounds such as inorganic phosphate salts while also stabilizing the ion containing cyanide.
The '686 patent illustrates a solution which comprises essentially an aqueous solution having an alkali metal ferrocyanide or ferricyanide and an organic film-forming material such as polyacrylic acid. In addition, a buffering agent, such as trisodium phosphate, is introduced to adjust the pH of the solution to a value within the range of about 3.0 to about 7.25.
However, and despite the ability of the aforementioned solutions to desensitize photoelectrographic masters, they are undesirable in that they are comprised of cyanide-containing compounds.
As indicated previously, cyanide-free conversion solutions have been proposed to eliminate the problems associated with such cyanide-containing solutions. Unfortunately, the cyanide-free desensitizing solutions employed to date have been unable to produce completely satisfactory lithographic printing plates or copy masters. Particularly, the cyanide-free solutions have resulted in a slower rate of hydrophilic film formation than the prior cyanide compound-containing solutions, and have not been capable of forming an immediately printable, hydrophilic film of sufficiently high physical strength after only one pass through an etch processor. As a result, scum often forms on the nonimage area or insufficient edge acuity of halftone dots is present.
Phytic acid (inositol hexaphosphate) esters and metal derivatives thereof have been known to form chelate compounds with metals, and previously have been proposed for use as desensitizers in the processing of the offset printing master as described in U.S. Pat. No. 3,592,640. However, these compounds have such a slow film forming rate that the desired hydrophilic film is not obtainable by a single pass through the processor and may often cause scum on the nonimage area or insufficient edge acuity of halftone dots.
In previously filed Japanese Patent Publication No. 5799/83, use has been proposed of an ion complex of a water-soluble cationic polymer and a mono- or divalent metal salt of an inositol hexaphosphate ester (e.g., sodium phytate, potassium phytate or calcium phytate). This complex has improved water retention but does not achieve a satisfactory etching speed. At the same time, the complex is highly dependent on moisture and is so sensitive to the printing environment that scum easily forms on the nonimage area during printing at low humidity.
In U.S. Pat. No. 4,579,591 there is disclosed a conversion solution which includes (a) at least one member selected from the group of ammonium and amine salts of a phytic acid ester, (b) a low molecular weight electrolyte, and (c) a water-soluble cationic polymer. According to that patent, an ion complex is formed between the water-soluble cationic polymer and the ammonium or amine salt of the phytic acid ester, which acts as the active species in the formulation.
Despite the development of the various cyanide-free conversion formulations, as discussed above, none have attained widespread commercial acceptance because of their inability to achieve sufficiently satisfactory results in comparison with the cyanide-containing formulations. Therefore, a need remains for a cyanide-free formulation having desensitizing properties substantially equal to such formulations which do contain cyanide compounds.